In view of a rapid increase in the traffic amount of a backbone network due to the spread of the Internet, there is a strong demand for implementing an ultra-high-speed long-distance optical communication system exceeding 100 Gbps. As a technique for implementing such an ultra-high-speed long-distance optical communication system, an optical phase modulation method utilizing a digital signal processing technique and a polarization demultiplexing technique are paid attention to.
The polarization demultiplexing technique is such that in an optical transmitter, two independent optical signals, whose frequency bands of carrier waves are the same as each other and whose polarization states are orthogonal to each other, are multiplexed. Further, the polarization demultiplexing technique is a technique such that in an optical receiver, the two independent optical signals are separated from a receiving signal so as to increase the transmission speed by two times. Combining the polarization demultiplexing technique, and a technique utilizing an optical phase modulation method such as QPSK (Quadrature Phase Shift Keying) makes it possible to implement an ultra-high-speed long-distance optical communication system as fast as 100 Gbps. A technique such that a process of compensating for an optical carrier wave frequency deviation and an optical phase deviation, and a polarization separation process of separating a signal into two independent optical signals are implemented by a digital signal processing circuit mounted on an LSI (Large Scale Integration) or the like for high-precision demodulation is called an optical digital coherent communication method.
Further, NPL 1 discloses a technique such that in a polarization demultiplexing optical communication system using an optical phase modulation method and a polarization demultiplexing technique, optical noise resulting from a nonlinear optical effect on an optical signal during optical fiber transmission is reduced. The technique disclosed in NPL 1 imparts time-wise optical phase changes different from each other with respect to two independent optical signals in an optical transmitter so as to compensate for the imparted time-wise optical phase changes in an optical receiver. This makes it possible to reduce optical noise resulting from a nonlinear optical effect generated between two independent optical signals, and to enhance the transmission characteristics.